1. Field of the Invention
The present invention relates to seismic isolation technique. More particularly, the invention relates to gravitational sliding pendulum systems.
2. Description of the Prior Art
The inventions entitled "Seismic Isolator" and "Multi-Step Base Isolator" (U.S. Pat. Nos. 4,974,378 and 5,056,280) incorporate ball transfer units which, due to their relatively small surface of contact with the pedestal plate, confine the bearing capacity of these types of isolators. Besides, the horizontal diameter of the pedestal plate heavily depends on the anticipated amplitudes and other characteristics of a possible earth vibration, and can be neither determined accurately because of probabalistic nature of the problem, nor sometimes manufactured to the full extent due to various kinds of imposed restrictions.
To prevent sliders from going out of the working surface of the pedestal plates, the current practice suggests enclosing cylinders (U.S. Pat. Nos. 4,644,714 and 4,883,250) with vertically projecting walls which inevitably cause heavy impacts in the case of contact and therefore are a remedy that is worse than the problem at hand.
To provide an adequate vertical bearing capacity, some kinds of expansion pads can be used, however they are not compatible with the gravitational type of sliding pendulum systems. A known type of seismic bearing that can, allegedly, keep the vertical reactions centered, is the Earthquake Protective Column Support (U.S. Pat. No. 4,644,714), however, it is not intended to slide on surfaces other than spherical.